1. Field of the Invention
The present invention relates to a laser oscillating apparatus for generating a laser beam by introducing an electromagnetic wave from a waveguide into a laser tube through a plurality of fine gaps formed in the waveguide wall and, more specifically, to a laser oscillating apparatus using a microwave as an electromagnetic wave for exciting a laser gas, an exposure apparatus using the same, and a device fabrication method.
2. Description of the Related Art
Recently, a so-called excimer laser has attracted attention as the only high-output laser which oscillates in the ultraviolet region. This excimer laser is expected to be applied to the electronic, chemical, and energy industries, particularly processing and chemical reactions of metals, resins, glass, ceramics, and semiconductors.
The principle of function of an excimer laser oscillator will be described below. First, laser gases such as Ar, Kr, Ne, F2, He, Xe, and Cl2 contained in a laser tube are excited by electron beam irradiation or discharge. Excited F atoms bond to inert Kr and Ar atoms in the ground state to generate KrF and ArF as molecules existing only in an excited state. These molecules are called excimers. Since excimers are unstable, they immediately emit ultraviolet rays and fall to the ground state. This phenomenon is called spontaneous emission. An excimer laser oscillator uses this to amplify as an in-phase beam in an optical resonator constructed of a pair of reflecting mirrors and extract as a laser beam.
In the case of excimer laser emission, an excitation source using microwaves is known as a laser gas exciting source as described above. Microwaves are electromagnetic waves having an oscillation frequency of a few hundred MHz to several tens of GHz. As a laser gas exciting method using this microwave, a method has been proposed by which a microwave is introduced from a waveguide into a laser tube through a gap (slot) formed in the waveguide wall, thereby exciting a laser gas in the laser tube into a plasma.
In this excitation method, even if the intensity distribution of microwaves emitted through the slots is uniform, a slot array structure in which a plurality of slots are arrayed in the long-axis direction of a resonator must be formed in order to supply a microwave to a long space meeting the resonator length of a laser beam. This structure is shown in FIG. 55. Referring to FIG. 55, a plurality of fine gaps (slots) 9202 are formed at equal intervals in a waveguide wall 9201. For convenience, the interior of a laser tube is schematically shown as an emission space.
When this slot array structure is used, regions (hatched elliptic regions in FIG. 55) between adjacent slots 9202 are necessarily microwave non-irradiation regions. Accordingly, when a laser gas existing in the emission space is to be excited by a microwave, the existence of these non-irradiation regions produces variations in the microwave intensity. This generates plasma discharge having a nonuniform distribution as a whole.
As described above, it is difficult to uniformize the radiation characteristic of an electromagnetic wave from a slot formed in a waveguide wall in an entire region over the slot. Usually, the distribution is a sinusoidal distribution in the slot long-axis direction or a similar distribution. That is, as shown in FIG. 56A, an electric field intensity distribution in the center along the slot long-axis direction is largest, and the field intensity distribution at the ends in the slot long-axis direction is smallest.
Additionally, as shown in FIG. 56B, an excited plasma has a property of concentrating to the center in the slot long-axis direction with respect to the microwave field intensity distribution. This promotes the nonuniform distribution of the field intensity in the slot long-axis direction. This is a great cause of preventing a uniform distribution of a plasma excited in the slot longitudinal direction.
This phenomenon is caused by the property that a plasma is easily excited in a central position along the slot longitudinal direction because the intensity of an electromagnetic wave as an excitation source is a maximum in this central position, and by the property that the excited plasma readily concentrates into the form of a sphere having the smallest surface area. This plasma excited in the central position forms a region having a low spatial impedance in the center of the slot. This portion preferentially consumes energy. Also, the plasma functions as a shield to reduce the slot length, which is designed to be the one by which a microwaves is emitted, to half that required by a microwave. This makes it difficult to emit a microwave outside the slot. By these two factors, a plasma is readily formed only in the center of a slot, and it is very difficult to excite a uniform plasma over the slot.
Furthermore, a plasma is generated immediately close to the microwave emission surface over a slot. However, in a plasma sheath formed over the slot a microwave can propagate. As a consequence, a microwave extends in the slot short-axis direction via this sheath region and disperses the input power. This makes it impossible to satisfy energy density necessary to excite an excimer laser. The reason for this is that when a plasma is diffused in a wide space, energy used to generate the plasma disperses, and this makes it difficult to realize enough energy density to excite an excimer.
FIG. 57 is a schematic view showing the way a microwave propagates via a plasma sheath. That is, FIG. 57 is a sectional view taken along a direction perpendicular to the longitudinal direction of a slot.
Although a plasma is not electrically grounded over the slot, the outside of the plasma is basically grounded directly to a waveguide 1. This results in a sheath potential difference and a sheath width difference between them. Therefore, if the plasma density is insufficient, the thickness of the sheath increases, and this results in easy nonuniform outward leakage of a microwave. Consequently, the plasma is thin immediately above the open end of the slot and thick on the outside.
The present invention has been made in consideration of the above situation, and has as its object to provide a laser oscillating apparatus which uses a slot array structure and yet realizes electromagnetic wave radiation uniform as a whole over the length of a laser tube and allows uniform laser emission with minimum energy loss, a high-performance exposure apparatus including this laser oscillating apparatus, and a high-quality device fabrication method using this exposure apparatus.
It is another object of the present invention to provide a laser oscillating apparatus which uses a slot array structure and yet suppresses diffusion of a plasma generated over a slot and allows uniform laser emission with minimum energy loss, a high-performance exposure apparatus including this laser oscillating apparatus, and a high-quality device fabrication method using this exposure apparatus.
A laser oscillating apparatus for achieving the above objects according to the present invention is a laser oscillating apparatus for exciting a laser gas in a laser tube by introducing an electromagnetic wave from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the laser gas, wherein the fine gaps and a wall of the laser tube are spaced apart by a predetermined distance to form an electromagnetic wave passage.
According to one aspect of the laser oscillating apparatus of the present invention, the distance from the fine gaps to the laser tube wall is an integral multiple of the half-wave length of an electromagnetic wave introduced from the waveguide.
According to another aspect of the laser oscillating apparatus of the present invention, an electromagnetic wave introduced from the waveguide is a microwave.
According to still another aspect of the laser oscillating apparatus of the present invention, a conductor is so formed as to surround the passage including the fine gaps, and the passage is an air gap with a predetermined width.
According to still another aspect of the laser oscillating apparatus of the present invention, the air gap is filled with a dielectric member.
According to still another aspect of the laser oscillating apparatus of the present invention, the width of the air gap is an integral multiple of the half-wave length of an electromagnetic wave introduced from the waveguide.
According to still another aspect of the laser oscillating apparatus of the present invention, only a distal end portion of the air gap is narrowed, and the air gap has the shape of a slit over the length of the laser tube in a portion where the air gap is in contact with the laser tube.
According to still another aspect of the laser oscillating apparatus of the present invention, the air gap is widened only in the vicinity of a distal end portion, and the width is substantially equal to the wavelength or the half-wave length of an electromagnetic wave introduced from the waveguide.
According to still another aspect of the laser oscillating apparatus of the present invention, the width near the distal end portion of the air gap changes along a longitudinal direction of the air gap by reflecting an intensity distribution of electromagnetic waves emitted from the fine gaps.
According to still another aspect of the laser oscillating apparatus of the present invention, dielectric lenses each having a symmetrical shape with respect to the fine gap are formed in the passage in at least a portion above the plurality of fine gaps.
According to still another aspect of the laser oscillating apparatus of the present invention, the waveguide is filled with a dielectric member.
In the laser oscillating apparatus described above, the fine gaps (slots) formed in the waveguide wall and the laser tube wall are spaced apart by a predetermined distance to form a passage for the electromagnetic wave. In this structure, the wavefront of an electromagnetic wave emitted from each fine gap is flattened near the laser tube wall, so the electromagnetic wave propagates in the form of an approximately plane wave as a whole in the laser tube. Accordingly, an electromagnetic wave in the form of a substantially uniform plane wave reaches a laser gas in the laser tube. This realizes uniform plasma discharge over the length of the laser tube and helps uniformize laser emission.
More specifically, the distance from the fine gaps to the laser tube wall is an integral multiple of the half-wave length, in the tube, of an electromagnetic wave introduced from the waveguide. Consequently, an electromagnetic wave emitted from each fine gap reaches the laser tube without interfering with a reflected wave or weakening each other.
Also, a conductor is so formed as to surround the passage including the fine gaps, and the passage is formed as a gap having a predetermined width (preferably, an integral multiple of the half-wave length of the electromagnetic wave in the tube). This can minimize energy loss. Preferably, the width is an integral multiple of the half-wave length of the electromagnetic wave in the tube. This makes it possible to give resonance conditions in a direction perpendicular to the fine gaps and set a high electric field in the slit.
Furthermore, by filling the gap with a dielectric member it is possible to prevent the generation of a plasma in the passage and reliably bring about plasma discharge only in the laser tube.
A laser oscillating apparatus for achieving the above objects according to the present invention is a laser oscillating apparatus for exciting a laser gas in a laser tube by introducing an electromagnetic wave from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the laser gas, wherein the width of longitudinal end portions of the fine gap is made larger than the width of a central portion thereof.
According to still another aspect of the laser oscillating apparatus of the present invention, the end portions have circular shapes with a diameter larger than the width of the central portion.
A laser oscillating apparatus according to the present invention is a laser oscillating apparatus for exciting a laser gas in a laser tube by introducing an electromagnetic wave from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the laser gas, wherein the fine gaps are formed apart from a central axis along a longitudinal direction of the waveguide, and each of the fine gaps is curved such that end portions are closer to the central axis than a central portion.
According to still another aspect of the laser oscillating apparatus of the present invention, the electromagnetic wave is radiated from the waveguide in the direction of a long end face of the waveguide.
A laser oscillating apparatus according to the present invention is a laser oscillating apparatus for exciting a laser gas in a laser tube by introducing an electromagnetic wave from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the laser gas, wherein an air-gap structure is formed in the waveguide wall in which the fine gaps are formed.
According to still another aspect of the laser oscillating apparatus of the present invention, in the vicinities of end portions of the fine gap, the air-gap structure is formed within a range from the end portions to a distance of xcexg/4 (xcexg is the waveguide wavelength of the electromagnetic wave).
According to still another aspect of the laser oscillating apparatus of the present invention, in the vicinities of end portions of the fine gap the air-gap structure is formed within a range from the end portions to a distance of xcexg/2 (xcexg is the waveguide wavelength of the electromagnetic wave).
According to still another aspect of the laser oscillating apparatus of the present invention, an air-gap portion of the air-gap structure in a central portion of the fine gap is made smaller than an air-gap portion near end portions of the fine gap.
According to still another aspect of the laser oscillating apparatus of the present invention, in a direction perpendicular to a longitudinal direction of the fine gap, the air-gap structure is formed with a width equal to an integral multiple of xcexg/2 (xcexg is the waveguide wavelength of the electromagnetic wave).
A laser oscillating apparatus for achieving the above objects according to still another aspect of the present invention is a laser oscillating apparatus for exciting a laser gas in a laser tube by introducing an electromagnetic waves from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the laser gas, wherein a dielectric lens or a structure substantially equivalent to a lens, for the electromagnetic wave, is formed in the waveguide in a portion close to the fine gap.
A laser oscillating apparatus according to the present invention is a laser oscillating apparatus for exciting a laser gas in a laser tube by introducing an electromagnetic waves from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the laser gas, wherein the width of longitudinal end portions of the fine gap is made smaller than the width of a central portion thereof.
A laser oscillating apparatus for achieving the above objects according to the present invention is a laser oscillating apparatus for exciting a laser gas in a laser tube by introducing an electromagnetic wave from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the laser gas, wherein the fine gap is formed in a portion where an emission characteristic of an electromagnetic wave depending on the fine gap is contrary to an intensity distribution of an electromagnetic wave propagating in the waveguide.
According to still another aspect of the laser oscillating apparatus of the present invention, the fine gap is formed such that a minimum value of an intensity distribution of an electromagnetic wave propagating in the waveguide is in substantially the center of the fine gap.
According to still another aspect of the laser oscillating apparatus of the present invention, the fine gaps are formed in a line at a pitch equal to the waveguide wavelength or the half-wave length of an electromagnetic wave in the waveguide.
According to still another aspect of the present invention, an electromagnetic wave introduced from the waveguide is a microwave.
In the laser oscillating apparatus of the present invention described above, each fine gap is formed in a portion where the emission characteristic of an electromagnetic wave from this fine gap is contrary to the intensity distribution of an electromagnetic wave propagating in the waveguide. The emission characteristic of an electromagnetic wave depending upon the fine gap exhibits a distribution in which, as described above, the electromagnetic wave intensity is a maximum in the central portion of the fine gap and decreases toward the end portions of the fine gap. Therefore, when each fine gap is formed in a position where the intensity distribution of an electromagnetic wave propagating in the waveguide is contrary to this, the intensity distribution of the electromagnetic wave propagating in the waveguide is influenced by the intensity distribution due to the emission characteristic of an electromagnetic wave depending on the fine gap. Consequently, the uniformity of the intensity distribution of an electromagnetic wave emitted from each fine gap actually increases over the whole region of each fine gap.
More specifically, when fine gaps are to be formed in a line at a pitch equal to the guide wavelength, or its half-wave length, of an electromagnetic wave in the waveguide by assuming an E-plane antenna, each fine gap is so formed that a minimum value of the intensity distribution of an electromagnetic wave propagating in the waveguide is positioned in substantially the center of the fine gap. That is, these fine gaps are formed in portions where they are equally deviated by xcexg/4 (xcexg: the waveguide wavelength) from positions at which a maximum value of the emission intensity distribution of an electromagnetic wave comes to the center of each fine gap in accordance with the intensity distribution of an electromagnetic wave propagating in the waveguide. By this relatively simple method, it is possible to further uniformize the intensity distribution of an electromagnetic wave emitted from each fine gap.
A laser oscillating apparatus for achieving the above objects according to still another aspect of the present invention is a laser oscillating apparatus for generating a plasma by exciting a laser gas in a laser tube by introducing an electromagnetic wave from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the plasma, comprising a shielding structure in the laser tube in order to prevent the plasma excited above the fine gaps from diffusing from a predetermined region.
According to still another aspect of the laser oscillating apparatus of the present invention, the shielding structure is formed to prevent diffusion of the electromagnetic wave and the plasma in a direction perpendicular to a longitudinal direction of the fine gaps.
According to still another aspect of the laser oscillating apparatus of the present invention, the shielding structure comprises a metal wall spaced apart from the fine gaps by a predetermined distance.
According to still another aspect of the laser oscillating apparatus of the present invention, the shielding structure is made from a mesh-like plate member.
According to still another aspect of the laser oscillating apparatus of the present invention, the shielding structure comprises a plurality of nozzles having predetermined openings.
According to still another aspect of the laser oscillating apparatus of the present invention, the nozzle is a passage of the laser gas.
According to still another aspect of the laser oscillating apparatus of the present invention, the shielding structure is formed by a magnetic field.
A laser oscillating apparatus for achieving the above objects according to still another aspect of the present invention is a laser oscillating apparatus for generating a plasma by exciting a laser gas in a laser tube by introducing an electromagnetic wave from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the plasma, wherein the width in a short-side direction of the fine gap is made smaller than the thickness of a sheath serving as a passage of the electromagnetic wave extending from an opening of the fine gap in the short-side direction.
According to still another aspect of the laser oscillating apparatus of the present invention, the width in the short-side direction is 10 to 100 xcexcm.
A laser oscillating apparatus according to still another aspect of the present invention is a laser oscillating apparatus for generating a plasma by exciting a laser gas in a laser tube by introducing an electromagnetic wave from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the plasma, wherein each of the fine gaps comprises a plurality of rows of slits.
According to still another aspect of the laser oscillating apparatus of the present invention, the width of slits in end rows is smaller than the width of slits in rows near the center.
According to still another aspect of the laser oscillating apparatus of the present invention, the length of slits in end rows is smaller than the length of slits in rows near the center.
According to still another aspect of the laser oscillating apparatus of the present invention, a shielding structure for suppressing diffusion of the plasma is formed on the side of an opening edge of the fine gap facing the laser tube.
A laser oscillating apparatus for achieving the above objects according to still another aspect of the present invention is a laser oscillating apparatus for exciting a laser gas in a laser tube by introducing an electromagnetic wave from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the laser gas, comprising a pair of waveguides formed to sandwich the laser tube such that formation surfaces of the fine gaps oppose each other, identical electromagnetic waves being supplied to the pair of waveguides to excite a laser gas in two opposite directions in the laser tube. In this apparatus, the pair of waveguides are constructed such that intensity distributions of electromagnetic waves introduced therefrom are shifted from each other.
According to still another aspect of the laser oscillating apparatus of the present invention, the formation surfaces of the fine gaps are short-end faces of the waveguides, and the fine gaps are formed in a line at equal intervals in a longitudinal direction of the fine gaps.
According to still another aspect of the laser oscillating apparatus of the present invention, the waveguides are arranged such that fine gaps corresponding to each other between the opposing formation surfaces are shifted relative to each other by a predetermined distance.
According to still another aspect of the laser oscillating apparatus of the present invention, the apparatus further comprises phase adjusting means for shifting phases of electromagnetic waves supplied into the waveguides relative to each other.
According to still another aspect of the laser oscillating apparatus of the present invention, each of the waveguides comprises tuning means for tuning an electromagnetic wave.
According to still another aspect of the laser oscillating apparatus of the present invention, an electromagnetic wave introduced from the waveguide is a microwave.
In the laser oscillating apparatus of the present invention described above, a pair of waveguides are so formed as to sandwich the laser tube such that the formation surfaces of the fine gaps (slots) oppose each other, and laser gas excitation is performed in two opposite directions in the laser tube. In accordance with various demands, the spatial positions of the two waveguides are adjusted such that the fine gaps in the opposing formation surfaces are shifted a predetermined distance relative to each other. Alternatively, the phases of electromagnetic waves supplied to these waveguides are shifted relative to each other. Consequently, variations of the wavefront of an electromagnetic wave resulting from the discontinuity of the fine gaps compensate for each other, so the light emission region substantially increases to flatten the wavefront. Accordingly, an electromagnetic wave substantially uniformly reaches the whole of a laser gas in the laser tube. This realizes uniform plasma discharge over the length of the laser tube and helps uniformize laser emission.
According to still another aspect of the present invention, the above laser oscillating apparatus is an excimer laser oscillator in which the laser gas is at least one inert gas selected from Kr, Ar, and Ne or a gas mixture of this inert gas and F2 gas.
Also, according to the present invention, there is provided an exposure apparatus comprising the aforementioned laser oscillating apparatus as a light source for emitting illuminating light, a first optical system for irradiating a reticle on which a predetermined pattern is formed with the illuminating light from the laser oscillating apparatus, and a second optical system for irradiating a surface to be irradiated with the illuminating light passing through the reticle, wherein the surface to be irradiated is exposed by projecting the predetermined pattern of the reticle.
Furthermore, according to the present invention, there is provided a device fabrication method comprising the steps of coating a surface to be irradiated with a photosensitive material, exposing the surface to be irradiated coated with the photosensitive material to a predetermined pattern by using the exposure apparatus described above, and developing the photosensitive material exposed to the predetermined pattern.
According to one aspect of the device fabrication method of the present invention, the surface to be irradiated is a wafer surface, and a semiconductor device is formed on the wafer surface.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.